I'm going to go out on a limb here; ISO/IEC 8859-1 does not define values between 126 and 159. See http://en.wikipedia.org/wiki/ISO/IEC_8859-1
this works;
var e = Encoding.GetEncoding("Windows-1252");
var s = e.GetString(new byte[] { 154 });
Console.OutputEncoding = Encoding.GetEncoding("Windows-1252");
Console.WriteLine(s);
I believe Windows-1252 is prefered;
...however the draft HTML 5 specification requires that documents advertised as ISO-8859-1 actually be parsed with the Windows-1252 encoding.[2])